Flat-film polymeric membranes can be manufactured by the method of thermally induced phase separation (TIPS) or nonsolvent-induced phase inversion (NIPI). The membranes that result are either isotropic or anisotropic, depending on conditions and processes employed for their manufacture. Isotropic membranes are uniform throughout in composition and structure, and typically a monolayer film produced by the TIPS process. Anisotropic membranes, on the other hand, are asymmetric and produced often by the NIPI process. These asymmetric membranes may consist of a layered structure, e.g., a thin dense layer supported by a thick porous nonwoven substrate. In particular, flat-film polymeric membranes, which are filled with a large quantity of particulates, can find a wide range of applications for use in filtration, separation and purification of gases and fluids, CO2 and volatile capture, vehicle emission control, energy harvesting and storage, device, support, protection, permeation, packaging, printing, and etc. However, the filled membranes often suffer from poor productivity and performance, due to complexities of their manufacture involving multiple components and processes.
U.S. Pat. Nos. 4,342,811 and 4,550,123 disclose a porous polypropylene (PP) fiber and film for use as protective clothing. The PP membranes contain 10 to 50 wt. % active carbons. U.S. Pat. Nos. 4,833,172 and 8,388,878 disclose a method for producing a porous ultrahigh molecular weight polyethylene (UHMWPE) film for breathable and printing applications. The UHMWPE film contains 65 to 90 wt. % SiO2. U.S. Pat. No. 5,964,221 discloses a CO2 absorbent device, which comprises a porous UHMWPE sheet containing 96 to 99.6 wt. % Ca(OH)2. These prior art membranes all are monolithic and symmetric in both structure and composition, composed either of a polymeric matrix of relatively low molecular weight (MW) or an extremely high content of inorganic particles. As a result, the membranes are often too low in strength or productivity for a commercially useful product. Moreover, the processed membranes yet retain a significant fraction of residual solvents, thereby greatly decreasing the performance of the particulate inclusions therein.
U.S. Pat. Nos. 4,877,679 and 5,032,450 disclose a multilayer absorbent polymeric membrane, produced by a discrete multistep process, i.e., laminating and/or coating one or more polymeric layers onto preformed microporous substrates. The preformed substrates are monolayer in structure and contain 50 to 90 wt. % SiO2. Again, these membranes disclosed are low in productivity due to their complex manufacturing processes. The membranes also lack strong adhesion between layers, even though it may be improved to an extent via surface treatment, adhesives, ultrasound, heat and pressure techniques, and the like. However, any additional process lowers productivity further, and alters or contaminates the interface that adversely affects properties of the resulting laminate. Yet, despite such efforts, the disclosed laminates can hardly develop sufficient adhesion when the joint consists largely of foreign inclusions. U.S. Pat. No. 6,893,483 discloses a two-layer adsorbent membrane, prepared by merely winding low and high density substrates alternately.
The prior art sorbent membranes, in general, suffer from a range of drawbacks, due to the nature of the structure and process. It is therefore highly desirable to provide a new method that can improve significantly both productivity and performance of flat-film sorbent polymeric membranes.